The discharged amount of selected gangue, coal lane gangue, rock lane gangue, hand-selected gangue of the coal mines and coal preparation plants as well as tailings of the coal preparation plants account for about 10-20% of total coal production. The production of gangue will increase along with the development of coal mining technology. At present, the annual discharged amount of gangue in China is more than 100 million tons, but its comprehensive utilization rate is less than 20%. 1.477 billion tons of gangue has been piled up to form a great number of gangue mountains covering an area of 13,300 hectares, the environment of mining areas are polluted by the spontaneous combustion, eruption and explosion, landslide, and leaching water of the gangue mountains. It is an effective method to prevent spontaneous combustion by paving soil on surface of the gangue mountain, compacting the soil and insulating the air path, such that the air inside the gangue mountain is exhausted and the fire is extinguished. The surface sealing and compaction method can be used for reducing the burning intensity and emission rate of pollutants, it is mainly used for controlling the fire behavior and pollution intensity of the gangue mountains, but it requires a timely maintenance. The method used in China with respect to stratified stacking gangue and compacting gangue with soil has achieved the desirable effects in preventing spontaneous combustion of the gangue mountain. However, the current practice of renovating and treating the gangue mountains by means of commercially available soil has many disadvantages, such as the transportation cost is high, the excavation of high-quality soil suffers from high cost and significant difficulty, in addition, the afforestation and reclamation effect are poor, and the water and soil erosion is serious. In addition, the washing of rain water on the fine gangue particles will form a large amount of silt, which pollute the water body; the sulfur in the gangue will be oxidized and react with the leaching water to form acid, which dissolve other toxic and harmful elements in the gangue, further pollutes the surrounding soil and water body; the discharged acidic water and dissolved ferric ions and other metal ions may cause suffocation of the organism, hardening of the river water, reducing the breeding areas of animals and plants and polluting the water source, thereby bring about grave consequences.
For example, Shenhua Ningxia Coal Industry Group Company in Northwest China consumes about 37 million tons of coal annually, and has by-product gasification slag and ash residue of about 4.8 million tons (dry slag). However, a large amount of stockpiled gasification furnace slag has to be buried and disposed by paying high expense resulting from the underdeveloped economy and the small scale of infrastructure in Ningxia Hui Autonomous Region, the used amount of building materials in the region is small, long-distance transportation is not cost-effective. On the one hand, the environmental protection benefits are poor, the coal group is required to pay a huge landfill fee of about 230 million yuan, it imposes a heavy burden on the enterprise; on the other hand, the rain leaching and filtrating (leaching, dissolving and infiltration) results in that the harmful elements in the slag are dissolved, when the harmful elements are infiltrate into the soil and absorbed by plants, it may contaminate surface water and underground water, thereby threat human health. Therefore, it is urgent to carry out research on the comprehensive utilization technology of gasifier slag and ash, solve the difficult problem of large-scale resource utilization of gasifier slag and ash, it is critical for the Shenhua Ningxia Coal Industry Group Company to gain both the environmental and economic benefits. The comprehensive utilization of gasification ash and slag, which make up a large share of solid wastes, is an important factor in achieving circular economy in the entire coal chemical project.
The process of mining and sorting other mineral resources also produce a large amount of waste residues, which are directly stockpiled in the dumping site or tailings pond, resulting in that a large quantities of land resources are occupied and the land resources cannot be effectively utilized. At the same time, the exposure of uncovered slag and tailings sediment causes the flying dust, which brings about significant inconvenience on the production of mines and the daily life of the nearby residents, resulting in a decline in the quality of the living environment. Moreover, when the slag is piled up in the dumping site, its front edge presents a high and steep slope, which is prone to be unstable when encountering with heavy rain, it will cause geological disasters such as collapse and landslide, and provide a material source for a formation of debris flow. In addition, the rain eluent also produces pollution of the heavy metal ions.